This invention relates to key telephone systems and more particularly to the control of key lamp illumination at key telephone sets.
The conventional key telephone set is a telephone set that has pick-up key access to one or more central office lines and, usually also, a hold button for placing any of the lines in the holding state. Each key of such key telephone sets typically has an illuminating lamp which is driven at a distinctive rate by auxiliary equipment to display to the user the status of the associated line, line circuit, whether off-hook, ringing or on hold. The most frequently used lamp illumination rates are, of course, steady illumination to depict the off-hook and picked-up state of a line to which the key telephone set has access, the flashing rate to indicate that the line is ringing and the wink rate to indicate that the line is on hold.
In my copending application filed of even date herewith entitled "A Universal PBX Line Circuit for Key and Non-Key Service," there is described an electronic line circuit which is capable of serving a line of a key telephone set on an "integrated" or on a conventional basis. When connected on the integrated basis all of the usual key telephone services such as ring tripping, common hold bridging and distinctive lamp rate driving are performed as part of the line port circuitry. When connected on the conventional basis the circuit merely gives access to the tip and ring leads of the communications bus and if key service is desired the customer must use a conventional relay-type auxiliary key equipment such as the Western Electric 400D line circuit described in the R. E. Barbato U.S. Pat. No. 3,436,488. Thus, it would be desirable to permit customers who desired to obtain the old style key line service to have their station set buttons associated with these lines to be powered by conventional relay art lamp interrupters and to intermix such lines on the same key telephone set with other telephone lines served by the more modern electronic key line circuits of the aforementioned copending application.
It is the usual circuit practice when driving a load from an interrupting source to insert the interrupter circuit in the ground lead so that the usual solid state interrupter switch may be driven by a low voltage or so-called logic level driving signal.
It is the standard practice in supplying power to key set lamps, however, to ground one side of the lamp load at or near the station set and to supply interrupted power at flash or wink rates to the other side of the lamp. This precludes the insertion of a solid state driver in the ground lead and with it the convenience of using logic level signals in the base or gate element of the driver. When the switching element is placed in the high side of the lamp power supply lead, a further problem arises because as many as 20 line lamps may be associated with a given line circuit in a key system and may have to be driven at the same time. Such switching of the high side of the lamp load may introduce severe switching transients. In the ordinary step-by-step or crossbar PBX using metallic crosspoints in the talking path such lamp switching transients can usually be ignored. In a time division PBX, however, where solid state crosspoints are employed, it is feared that lamp switching transients could constitute an undesirable noise pollutant because the lamp interruption rates tend to fall within the voice communications band. I have devised an arrangement for delivering lamp rate power which is compatible with existing key system lamp wiring practices and which nevertheless holds lamp transients to a minimum.
As has been mentioned, the lamp illumination supply may have to drive one, five or up to twenty key lamps. Fusing of the lamp power supply is therefore troublesome since the load cannot be predicted at the factory and it would seem that field conditions must determine the appropriate power supply voltage. It would accordingly be advantageous to have a lamp illumination supply that could deliver distinctive lamp illumination rates to different types of lamp loads without requiring field readjustment of the supply whenever the load was changed.